1. Field of the Invention
The invention relates to ink supply devices for printing apparatuses and, more particularly, to an ink supply device for supplying an ink layer over the outer circumferential surface of a rollerlike ink supply means.
2. Description of the Related Art
As shown in FIG. 3, a known ink supply means for printing apparatuses includes: a squeegee roller 101 supported by a shaft between side plates that confront each other so as to be rotatable about the central axis of the shaft thereof; a distributor 102, arranged above the squeegee roller 101, for supplying ink over the outer circumferential surface of the squeegee roller 101; and a doctor roller 103, arranged next to the squeegee roller 101 while interposing a predetermined distance therebetween, for controlling the thickness of an ink layer formed over the outer circumferential surface of the squeegee roller 101. The distributor 102 is arranged above the squeegee roller 101, senses the amount of ink in an ink pool 104 through an ink amount sensing needle 105a of an ink sensor 105, and supplies the ink in response to a signal indicating excess or deficiency of the ink. It may be noted that the distributor 102 can be formed into various shapes. For example, a distributor 102 having a plurality of small-diameter holes formed in a tubular member running in parallel with the squeegee roller 101, or a distributor 102 having a plurality of nozzles may be applicable.
The ink supply device thus constructed has the ink pool 104 that is arranged in cooperation with a part of the squeegee roller 101, a part of the doctor roller 103, and the side plates through which the squeegee roller 101 is journaled. A vortex that fluidly rotates about an axis running substantially in parallel with the squeegee roller 101 when the squeegee roller 101 starts to rotate is produced in the ink pool 104. While the squeegee roller 101 is rotating, the ink in the ink pool 104 is applied over the outer circumferential surface of the squeegee roller 101 through a small gap between the squeegee roller 101 and the doctor roller 103 to thereby allow the ink to be transferred onto the inner circumferential surface of a plate cylinder that takes the form of a circular cylinder, printing paper, and the like.
When large quantities of ink are consumed because it is immediately after the ink has been supplied from the distributor 102 or when large quantities of ink are consumed locally because an image to be printed has dark portions localized, a ink lump portion M is produced on the outer circumferential surface of the squeegee roller 101 or on the ink pool 104 as shown in FIG. 3. If such ink lump portion is immediately spread uniformly along the length of the squeegee roller 101, there is no problem. However, the viscosity of the ink used for this type of ink supply device is comparatively high and it, therefore, takes some time before the ink is spread out.
In addition, the aforementioned conventional ink supply device is not designed to vary the rotational speed of the squeegee roller 101 even if the ink has been supplied. Therefore, the ink lump portion M directly affects the printing, causing an irregularly printed image and the like, as a result of the ink lump portion M having been transferred onto the inner circumferential surface 106 of the cylindrical drum. Further, when the ink amount sensing needle 105a comes in contact with the ink lump portion M under the condition that the amount of ink is deficient in terms of the entire part of the ink pool 104, the ink amount sensor judges that the amount of ink is sufficient to stop the supply of ink, causing shortage of ink. In addition, if the ink amount sensing needle 105a and an overflow sensing needle 105b are positioned at a portion other than which is the ink lump portion M in the ink pool 104, and in which the amount of ink is small, the sensor judges that the amount of ink is deficient and thereby continues the supply of ink. As a result, the size of the ink lump portion M is increased, bringing about an overflow of ink.